As a prior art technology, a sustained-release preparation comprising a drug, a polylactic acid and a glycolic acid/hydroxycarboxylic acid [HOCH(C.sub.2 -C.sub.8 alkyl)COOH] copolymer is described in EP-A-481,732, for instance. As a production method for said preparation, the in-water drying method is described in which a w/o emulsion, comprising an aqueous solution of a physiologically active peptide as an internal aqueous phase and an organic solvent solution of a biodegradable polymer as an oil phase, is added to water or the like to yield a w/o/w emulsion, from which sustained-release microspheres are produced.
Also, a production of microcapsules using a water-soluble drug and a polymer by the in-water drying method is described in Japanese Patent Unexamined Publication Nos. 100516/1985 (EP-A 145240) and 201816/1987 (EP-A 190833).
In in-water drying, insufficient solvent removal, due to the unsatisfactory speed of solvent removal from microspheres, is likely to cause sphere aggregation, resulting in problems regarding the dispersibility of spheres and the needle passability during administration. An attempt to achieve sufficient solvent removal results in significantly extended in-water drying time, which in turn decreases the drug entrapment ratio in the microspheres obtained and cannot bring satisfactory results.